Magnetoresistive Random Access Memory (MRAM) has a read function based on a tunneling magnetoresistive (TMR) effect in a MTJ stack wherein a tunnel barrier is formed between a free layer and a reference layer. The free layer serves as a sensing layer by switching the direction of its magnetic moment in response to external fields (media field) while the reference layer has a fixed magnetic moment. The electrical resistance through the tunnel barrier (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby provides an electrical signal that is representative of the magnetic state in the free layer. A spin-transfer torque (STT) MRAM involves a spin polarized current to switch the free layer, and is described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996). J-G. Zhu et al. has described another spintronics device called a spin transfer oscillator (STO) in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA) are preferred over their counterparts that employ in-plane anisotropy because the former has an advantage in a lower writing current for the same thermal stability, and better scalability for higher packing density which is one of the key challenges for future MRAM applications. For memory applications as in a STT-MRAM, the free layer magnetization direction is expected to be maintained during a read operation and idle, but change to the opposite direction during a write operation if the new information to store differs from its current memory state. The ability to maintain free layer magnetization direction during an idle period is called data retention or thermal stability. Moreover, the reference layer is subject to the same spin polarized current applied to the free layer and must be thermally stable. A higher PMA in the reference layer (and free layer) is associated with improved thermal stability.
In many cases, a CoFeB/MgO interface is used to induce interfacial perpendicular anisotropy and PMA in one or both of a CoFeB reference layer and CoFeB free layer when MgO is the tunnel barrier. Reference layers are often based on face centered cubic (FCC) multilayers or materials having a <111> crystal structure. Thus, it is important to be able to promote a FCC <111> texture in the reference layer in order to attain the best magnetic properties. Furthermore, the reference layer should have a uniform film thickness with a smooth top surface to provide uniformity in the overlying tunnel barrier and free layer that will in turn enable a high magnetoresistive (MR) value and more reproducible tunnel barrier properties from one MTJ to the next.
Typically, a seed layer is deposited as an underlayer to promote good crystal growth in an overlying reference layer, and is usually selected to satisfy one or two requirements including low cost, excellent film uniformity, promotion of a <111> crystal structure in the reference layer, and good diffusion resistance to prevent materials such as Ta from the bottom electrode (BE) or an underlayer from migrating to the tunnel barrier and degrading the MTJ performance. An improved seed layer is needed that satisfies all the aforementioned requirements to enhance PMA in the overlying reference layer and enable optimum tunnel barrier function.